The Advanced Television Systems Committee (ATSC) publishes standards for encoding digital television (DTV) signals. For example, Section 5.1 of ATSC A/53E “Digital Television Standard,” Annex D, dated 27 Dec. 2005, describes an encoding process in which:                Incoming data is randomized and then processed for forward error correction (FEC) in the form of Reed-Solomon (RS) coding (20 RS parity bytes are added to each MPEG-2 packet), one-sixth data field interleaving and two-thirds rate trellis coding.        
FIG. 3 illustrates a conventional ATSC transmitted side encoder structure 300. As shown in FIG. 3, MPEP data 302 to be transmitted enters the encoder structure 300 and is randomized by randomizer circuitry 304. Reed-Solomon encoder circuitry 306 generates Reed-Solomon error correction codes, and the thus-encoded transmission data is provided to convolutional interleaver circuitry 308. The output of the convolutional interleaver circuitry 308 is provided to trellis encoder circuitry 310, the output of which is then provided 312 to the transmission channel.
FIG. 4 is a block diagram illustrating a conventional trellis encoder structure, as a component of the FIG. 3 ATSC transmitted-side encoder structure. The 8 VSB transmission sub-system employs a 2/3 rate (R=2/3) trellis code (with one unencoded bit which is precoded by a precoder 402). That is, one input bit (X1) is encoded into two output bits (Z1 and Z0) using a 1/2 rate convolutional code while the other input bit (X2) is precoded. A 4-state trellis encoder 404 is used. The output of the trellis encoder (Z2, Z1 and Z0) is provided to a symbol mapper 406, which is output to the channel (312, in FIG. 3).
It is desired to accurately and efficiently decode such encoded data.